VESPA GTS Super 300 ie 2008 Service Station Manual page 168

Engine
Studs
Check that the stud bolts have not worked loose
from their seat in the crankcase.
Check the depth of stud bolt driving with a gauge,
as indicated in the picture. If it varies significantly
from the driving depth indicated, it means that the
stud bolt has yielded.
In this case, replace it.
ENG - 168
Vespa GTS Super 300 ie (2008)